Little precise knowledge is available concerning the terminal distribution of parasympathetic fibers to discretely localized portions of the heart. There is little appreciation for the fact that selective excitation of vagal fibers, in theory at least, could exert highly restricted influences upon sinoatrial (SAN), atrioventricular (AVN) or other cardiac tissues. Yet many bradydysrhythmias may be directly implemented by disturbances within the central nervous system. Physiologic reflexes (baroreceptor, respiratory, pain, emotion, etc.) could function through activation of autonomic fibers supplying automatic, conductile, contractile, and vascular tissues of the heart. The proposed experiments will examine the separate projections of right and left vagi to the right and left hearts, with particular emphasis on local functional responses to excitation of small, individual branches. Recordings will be made from electrical and mechanical transducers signaling activity within SAN, AVN, right and left artial muscle, and internodal conduction pathways, while electrically stimulating a single vagus or its small branches. Acute, anesthetized dogs will be subjected to cardiac decentralization to prevent reflex activation of other neural inputs. In a third protocol, the enzyme, horseradish peroxidase (HRP) will be injected in and around the SAN and AVN regions where parasympathetic postganglionic cell bodies are thought to reside. The enzyme will be absorbed by preganglionic terminals and transported by retrograde axoplasmic flow to their somata which may then be identified in the brainstem. The left vagus will be sectioned in one half the animals (with either SAN or AVN HRP injection) while the right vagus will be cut in the other half. This will indicate which vagus distributes the predominant efferent parasympathetic supply to each of these controlling regions of the heart. Once the primary concentrations of cells are located within the brainstem, electrodes will be inserted stereotaxically into them for electrical stimulation while recording local changes in electrical and mechanical activity in the heart. We expect to learn whether SAN discharges rates, AV conduction, and myocardial contractions may come under independent control of specific cell concentrations in the brain, and whether these regulatory regions of the heart are primarily controlled by right and/or left vagi.